The ‘simplest’ solar water heater ever, especially suited to those with little space or money.
Save empty soft drink 2–3 litre plastic bottles.
Paint the bottles black or cover with some dark fabric will help to collect heat. Secure the fabric with some elastic bands.
Fill the empty bottles with water.
Put the finished bottles in a sunny spot. The dash of a closed car works great. Behind glass on a windowsill works too. For extra power use something shinny like foil or a mirror to reflect the sun onto the bottle. After a few hours of sun you have enough warm water to do whatever, with no electricity being used.
Place the unit near a radiator in the wintertime and you can get warm water.